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J. Paleont., 82(2), 2008, pp. 255–266 Copyright 䉷 2008, The Paleontological Society 0022-3360/08/0082-255$03.00

LATE CRETACEOUS AND PALEOGENE FRESHWATER GASTROPODS FROM NORTHEASTERN MEXICO MARIÁ DEL CARMEN PERRILLIAT,1 FRANCISCO J. VEGA,1 BELINDA ESPINOSA,2

AND

EDNA NARANJO-GARCIA3

Instituto de Geologıá, UNAM, Ciudad Universitaria, Coyoacań, 04510 – Me´xico, D.F., ⬍[email protected]⬎; ⬍[email protected]⬎; 2 Beneme´rita Escuela Normal de Coahuila, Calzada de los Maestros s/n, Zona Centro, 25000 – Saltillo, Coahuila, Me´xico ⬍[email protected]⬎; and 3Instituto de Biologıá, UNAM, Ciudad Universitaria, Coyoacań, 04510 – Me´xico, D.F. ⬍[email protected]⬎ 1

ABSTRACT—Twenty-three species of Campanian, Maastrichtian, Paleocene, and Eocene freshwater gastropods from northeastern Mexico are described, including eight new species: Viviparus mcbridei, Pachychilus (Pachychiloides) lawtoni, Melanoides (Melanoides) yolandae, Melanoides (Melanoides) wollebeni, Physa cepedaensis, Mesolanistes magnus, Mesolanistes murrayi, and Gyraulus zoltani. Specimens were collected from fine-grained, green sandstone and mudstone, red mudstone, and as hematized remains in fine light-brown sandstones belonging to the following stratigraphic units of northeastern Mexico: Cerro del Pueblo (Campanian, Parras Basin), Olmos (Maastrichtian, Sabinas Basin), Las Encinas (Paleocene, Parras Basin), and Carroza (Eocene, La Popa Basin) Formations. All except two of the genera (?Pyrgulifera and Mesolanistes) have recent representatives. Review of habitats of living species of the six extant genera and interpretations of sample lithologies support the inference of dominantly freshwater paleoenvironments. Only one Maastrichtian locality is interpreted confidently to have had a brackish water influence.

INTRODUCTION

F

are found in Cretaceous through Eocene sediments in Mexico. However, systematic work on these gastropods has been limited. Vega and Perrilliat (1992) described three species of freshwater thiariid gastropods from the Adjuntas Formation (lower Eocene, La Popa Basin): Hemisinus (Longiverena) reticulosus Vega and Perrilliat, 1992, H. (L.) adjuntaensis Vega and Perrilliat, 1992, Melanatria ypresiana Vega and Perrilliat, 1992, and illustrate Melanatria sp. and ?Melanatria sp. From northwestern Mexico, Lucas et al. (1995) reported five species of nonmarine gastropods and seven species of nonmarine bivalves from the Campanian–Maastrichtian Cabullona Group in Sonora of northwestern Mexico. The gastropod species include Viviparus australis Kues, 1995, Campeloma aff. C. vetula (Meek and Hayden, 1856), Lioplacodes? sp., Physa aff. P. copei White, 1877, and Mesolanistes reesidei (Stanton, 1917). This study identifies the relatively abundant and diverse assemblages of freshwater gastropods from six localities in three sedimentary basins in northeastern Mexico (Parras, La Popa and Sabinas Basins; Fig. 1.1). Including the Eocene freshwater gastropods from the Adjuntas Formation (Vega and Perrilliat, 1992), there are now 29 species recorded for the Campanian through Eocene of northeastern Mexico (Table 1). RESHWATER SNAILS

MATERIALS AND METHODS

The gastropod specimens were collected as surface-exposed shells from six samples in four formations in northeastern Mexico. In the Cerro del Pueblo Formation (Difunta Group, Parras Basin), three localities (IGM 3491, IGM 3492, IGM 3495; Figs. 1.2, 2, 3.1) have recently been dated as late Campanian (Eberth et al., 2004). In the Olmos Formation (Sabinas Basin), one locality (IGM 3476; Figs. 1.3, 2, 3.2) is Maastrichtian (McBride et al., 1974). In the Las Encinas Formation (Difunta Group, Parras Basin), one locality (IGM 3494; Figs. 1.2, 2, 3.3) in red beds is Paleocene (Vega et al., 1999). And in the Carroza Formation (Difunta Group, La Popa Basin), one locality (IGM 3496; Figs. 1.4, 2, 3.4) in green beds is lower Eocene (Vega et al., 1989). Gastropods are abundant in green mudstones and fine-grained sandstone beds, less abundant but more diverse as hematized remains in fine-grained, light brown sandstone beds, and only a few species were found in red mudstones (Table 2). A summary of the described lithologies, paleoenvironments, and locality numbers for each species here described, plus the ones previously reported by Vega and Perrilliat (1992), is given in Table 2. Detailed sedimentological descriptions are given by McBride et al. (1974), and a stratigraphic correlation diagram based on the work of Soegaard et al. (2004) is given in Figure 2.

Preservation is in most specimens adequate for a confident identification. In some cases, specimens are slightly deformed but preserve enough morphological details to offer a description and comparison with similar species. In particular, viviparids found in red beds show different levels of deformation, but the large number of specimens permit us to have a confident idea of the original shape of the shell. Hematized gastropods have also some degree of deformation, but the species are also represented by many specimens, and description of the original shape is possible. Most of the remaining species here described are represented by specimens that are not deformed and have shell ornamentation, which allow a detailed description and identification. The studied material is deposited in the Instituto de Geologıá, Universidad Nacional Autońoma de Me´xico. Types are included in the Type Collection and classified under the acronym IGM. The localities are registered in the locality catalogue of the Instituto de Geologıá, Universidad Nacional Autońoma de Me´xico. PALEOENVIRONMENTS

Cerro del Pueblo Formation.⎯The lower part of the Campanian Cerro del Pueblo Formation (with a thickness of more than 700 m), exemplified by localities IGM 3491 in fine-grained, lightbrown sandstone and IGM 3492 in light green sandstone (Fig. 3.1), exhibits a sequence of mudstones and sandstones, representing cycles of marine, estuarine, and freshwater paleoenvironments. Paleoenvironment interpretations include deltaic lake and bay deposits (McBride et al., 1974; Eberth et al., 2004). Remains of marine invertebrates include ammonites (Sphenodiscus sp.) and oysters (Flemingostrea sp.) from dark-gray sandstones, 5 meters below locality IGM 3491. Locality IGM 3491 (here named General Cepeda 1) has light-brown, fine-grained sandstone beds that include hematized wood fragments, fruits, gastropods, bivalves, turtle bones, crocodile teeth, and coprolites representing freshwater deposits. The fruits (Rodrı´guez de la Rosa and CevallosFerriz, 1994) and the coprolites (Rodrı´guez de la Rosa et al., 1998) have been studied in detail. A diverse assemblage of freshwater gastropods includes 11 species in four genera: Viviparus sp., V. cf. V. australis Kues, 1995, Physa cepedaensis n. sp., P. cf. P. michaudi Matheron, 1842, P. cf. P. usitata, White, 1895, Mesolanistes murrayi n. sp., Bulinus sp. cf. B. atavus White, 1877, Bulinus sp. 1, Bulinus sp. 2, Bulinus sp. 3, Bulinus sp. 4, and Gyraulus zoltani n. sp. The very small G. zoltani can be found inside the M. murrayi n. sp. Three species of Mesolanistes (Upper Cretaceous) and one of Gyraulus (Lower Cretaceous) in continental red beds of China were interpreted as freshwater gastropods (Hengren et al., 1979). Mesolanistes has also been reported from Cretaceous freshwater deposits from Utah (Yen,

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JOURNAL OF PALEONTOLOGY, V. 82, NO. 2, 2008

FIGURE 1—1, Location map of northeastern Mexico, indicating geographic position of the Parras, La Popa and Sabinas Basins. Dark areas correspond to Figure 1.2 to 1.4, with position of studied localities. 2, Regional map of General Cepeda County, Coahuila, west of Saltillo, indicating position of localities IGM 3491 (General Cepeda 1), IGM 3492 (General Cepeda 2), IGM 3493 (Puerto Colorado), and IGM 3495 (Rincoń Colorado). 3, Regional map of Sabinas County, Coahuila, northwest of Monclova, indicating position of locality IGM 3476 (Rancho La Soledad). 4, Regional map of Mina County, Nuevo Leoń, northwest of Monterrey, indicating position of locality IGM 3496 (La Carroza).

1945). Recent species of Physa occur frequently in freshwater environments globally, feeding on algae and organic matter in ponds. Their populations can increase and spread rapidly (Naranjo-Garcıá, 2003; Schmitz and Andreasson, 2001). Eberth et al. (2004) interpreted facies at the Cerro del Pueblo Formation as pond deposits. Recent species of Gyraulus and Bulinus occur in fluvial channels and lacustrine environments. Gyraulus is not very abundant and is associated with the more diverse freshwater assemblages than is Bulinus, being found in the submerged portion of roots in lakes, pools, and swamps (Bandel, 1998; Genner et al., 2004; Plaziat and Younis, 2005). Bulinus lives on sub-aquatic plants, producing its highest densities at depths between 3.0 and 4.5 m, but Bulinus is no longer present among Recent American freshwater mollusks (Burch and Cruz-Reyes, 1987; Burch, 1989; Ramı´rez et al., 2003; Paraense, 1975). The presence of small prismatic iron oxide nodules suggests original precipitation of pyrite

under anoxic conditions, due to accumulation of organic matter in stagnant water, although alternatively the prisms could have been oxidized secondarily during the diagenetic process (de LeońDa´vila et al., 1999). Overall, the ecological preferences of extant relatives of the recovered fossil gastropods support the inference of deposition in freshwater ponds or lakes. Light-green, fine-grained sandstone beds are found 2 m above strata of locality IGM 3491 and are termed here as locality IGM 3492 (a.k.a. General Cepeda 2; Fig. 1.2). Abundant gastropods and few bivalves are found here. Traces of roots and stems occur in this horizon. Channel deposits associated with reducing conditions are interpreted for this locality. Five species of gastropods in four genera are found here: ?Pyrgulifera sp., Melanoides (Melanoides) yolandae n. sp., M. (M.) wollebeni n. sp., Tympanotonus (Tympanotonus) nodosa (Wolleben, 1977), and Mesolanistes magnus n. sp. The paleoenvironment of this locality is poorly

PERRILLIAT ET AL.—FRESHWATER GASTROPODS FROM MEXICO

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TABLE 1—Stratigraphic distribution of freshwater gastropods in Upper Cretaceous and Paleogene formations from northeastern Mexico. Age Stratigraphic unit Family/species

Campanian

Maastrichtian

Paleocene

Eocene

Cerro del Pueblo Formation

Olmos Formation

Las Encinas Formation

Carroza Formation Adjuntas Formation

Viviparidae Viviparus mcbridei new species Viviparus cf. V. leidyi (Meek and Hayden) Viviparus cf. V. raynoldsanus (Meek and Hayden) Viviparus cf. V. australis Kues Thiaridae Pachychilus (Pachychiloides) lawtoni new species Hemisinus (Longiverena) reticulosus Vega and Perrilliat Hemisinus (Longiverena) adjuntaensis Vega and Perrilliat Melanatria wyomingensis (Meek) Melanatria ypresiana Vega and Perrilliat Melanatria cf. M. ypresiana Vega and Perrilliat Melanatria sp. ?Melanatria sp. ?Pyrgulifera sp. Melanoides (Melanoides) yolandae new species Melanoides (Melanoides) wollebeni new species Naticidae Natica sp. Potamididae Tympanotonus (Tympanotonus) cretaceus (Wade) Tympanotonus (Tympanotonus) nodosa (Wolleben) Physidae Physa cepedaensis new species Physa cf. P. michaudi Matheron Physa cf. P. usitata White Mesolanistes magnus new species Mesolanistes murrayi new species Planorbidae Bulinus cf. B. atavus White Bulinus sp. 1 Bulinus sp. 2 Bulinus sp. 3 Bulinus sp. 4 Gyraulus zoltani new species

FIGURE 2—Chronostratigraphic correlation chart of Cretaceous and Paleogene stratigraphic units of the Parras, La Popa, and Sabinas Basins.

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

FIGURE 3—Stratigraphic sections in Coahuila state, Mexico. 1, Cerro del Pueblo Formation (Parras Basin). 2, Olmos Formation (Sabinas Basin). 3, Las Encinas Formation (Parras Basin). 4, Carroza Formation (La Popa Basin). Sections are not drawn to the same scale. Dotted white strata represent sandstone; gray strata represent red mudstone; dashed-line gray strata represent green and gray mudstone.

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TABLE 2—Paleoenvironmental distribution of freshwater gastropods in the reported localities. Lithology

Family/species

Light-green sandstone

Green mudstone

Red mudstone

Light-brown sandstone

Environment

Brackish water

Channels, streams

Delta-plain, pools

Ponds, lakes

IGM Locality

3476, 3492, 3495

1973, 3496

3494

2587, 3491

Viviparidae Viviparus mcbridei new species Viviparus cf. V. leidyi (Meek and Hayden) Viviparus cf. V. raynoldsanus (Meek and Hayden) Viviparus cf. V. australis Kues Thiaridae Pachychilus (Pachychiloides) lawtoni new species Hemisinus (Longiverena) reticulosus Vega and Perrilliat Hemisinus (Longiverena) adjuntaensis Vega and Perrilliat Melanatria wyomingensis (Meek) Melanatria ypresiana Vega and Perrilliat Melanatria cf. M. ypresiana Vega and Perrilliat Melanatria sp. ?Melanatria sp. ?Pyrgulifera sp. Melanoides (Melanoides) yolandae new species Melanoides (Melanoides) wollebeni new species Naticidae Natica sp. Potamididae Tympanotonus (Tympanotonus) cretaceus (Wade) Tympanotonus (Tympanotonus) nodosa (Wolleben) Physidae Physa cepedaensis new species Physa cf. P. michaudi Matheron Physa cf. P. usitata White Mesolanistes magnus new species Mesolanistes murrayi new species Planorbidae Bulinus cf. B. atavus White Bulinus sp. 1 Bulinus sp. 2 Bulinus sp. 3 Bulinus sp. 4 Gyraulus zoltani new species

defined, due to conflicting environmental affinities of fossil and extant species of the genera found here. Pyrgulifera glypta Avias and Rey, 1958, was reported from shallow-marine Campanian deposits from New Caledonia (Stilwell, 1994). Pyrgulifera humerosa White, 1876 from the Cretaceous Bear River Formation in Wyoming is found in non marine sediments. Melanoides feed on organic matter deposited in muddy bottoms of freshwater environments, and its feeding activity is important in nutrient recycling for aquatic plants. Reports of Recent Tympanotonus indicate that some species of this genus inhabit creeks of most Nigerian rivers (Ekpo and Uzegbu, 2004), while other authors have documented a brackish-water affinity for living and fossil species of this genus (Brown, 1994; Schmitz and Andreasson, 2001). As mentioned before, Late Cretaceous Mesolanistes from red beds of China have been interpreted as freshwater gastropods (Hengren et al., 1979). However, relative abundance of juvenile and adult stages of Tympanotonus (Tympanotonus) nodosa (Wolleben, 1977), together with presence of some typical marine forms such as ?Pyrgulifera (numerous records exist of Pyrgulifera from marine Cretaceous deposits of Wyoming and adjacent areas), and turritellid gastropods, suggest that the environment in this locality had brackish influence. Some studies, though, demonstrated that Late Cretaceous specimens of Pyrgulifera from Serbia were transported to environments differing from their original habitat (Banjac, 1992). Fossils occur in light-green and gray mudstone beds at locality

X X X X X X X X X X X X X X X X X X X X X X X

X

X X X X X X

IGM 3495 and contain plants, terrestrial arthropods (CifuentesRuiz et al., 2006) and dinosaur remains, as well as bivalves and the snail Melanatria wyomingensis (Meek, 1873). This locality is placed in the middle part of the Campanian Cerro del Pueblo Formation (Fig. 3.1) and represents interdistributary bay deposits according to Eberth et al. (2004). Recent representatives of Melanatria are reported from freshwater environments in Madagascar (Grossman, 1967; Ko¨hler et al., 2004). Olmos Formation.⎯The upper part of the Olmos Formation is represented by brown and green sandstone beds (Fig. 3.2). Dinosaur tracks are found in brown sandstone, which underlie lightgreen sandstones at locality IGM 3476 (Fig. 1.4) that contain numerous specimens of Tympanotonus (Tympanotonus) cretaceus (Wade, 1926). As discussed previously, members of this nonmarine gastropod genus prefer brackish influenced environments, and Recent populations of this genus are found in estuaries dominated by mangroves in Ghana and Cameroon (Gordon, 1997; Bandel and Kowalke, 1999). Las Encinas Formation.⎯Paleocene red beds of the upper Las Encinas Formation contain freshwater gastropods at locality IGM 3494 (Fig. 1.2), which is located 70 m above the base of the formation (Fig. 3.3). Murray et al. (1962) suggested that the Cretaceous-Tertiary transition should be placed within the upper part of the Las Encinas Formation. This was confirmed by the presence of Paleocene oysters in this interval (Vega et al., 1999). Four species of freshwater gastropods in two genera are present here:

PERRILLIAT ET AL.—FRESHWATER GASTROPODS FROM MEXICO Viviparus mcbridei n. sp., V. cf. V. leidyi (Meek and Hayden, 1856) (Paleocene of Montana and North Dakota), V. cf. V. raynoldsanus (Meek and Hayden, 1862) (Paleocene of Montana and North Dakota), and Melanatria cf. M. ypresiana Vega and Perrilliat, 1992 (lower Eocene, Nuevo Leoń). Specimens are found in red mudstones interpreted by McBride et al. (1974) as lake deposits. Lithology changes laterally to the south to coarsegrained sandstone, with small channels that include transported, poorly preserved freshwater gastropods. Recent and fossil species of Viviparus inhabit numerous freshwater environments (Prashad, 1928; van Damme, 1984; Gloër and Meier-Brook, 1998), with low tolerance to salinity. Viviparus can be found partially buried in the mud or silt of lakes, ponds, or slower portions of streams (Pace, 1973). They prefer low-energy conditions where there is some vegetation and a muddy substrate (Clench and Fuller, 1965). Carroza Formation.⎯Locality IGM 3496 (Fig. 1.3) is found 520 m above the base of the Carroza Formation (Fig. 3.4). The Carroza Formation (La Popa Basin) overlies the lower Eocene Viento Formation (Perrilliat and Vega, 2004), and is composed mainly of red beds. Green mudstone outcrops occur at the top upper part of the formation, interpreted as delta-plain deposits (McBride et al., 1974). Wood fragments and the gastropod Pachychilus (Pachychiloides) lawtoni n. sp. are contained in green mudstones of a 40-m-wide paleochannel, that change color laterally to red. Continental red beds of uppermost Cretaceous in China contain specimens of Pachychiloides (Hengren et al., 1979). Recent species of Pachychilus are distributed as neotropical fauna in freshwater environments, such as rivers and streams in eastern Mexico, the Panuco River system and Tamaulipas to southeastern Mexico (Hinkley, 1907; Pilsbry and Hinkley, 1910). SYSTEMATIC PALEONTOLOGY

The description of the new species and the discussion were prepared jointly by Perrilliat and Vega, and the new names should be attributed to them. Relative size scale for specimens is as follows: small ⫽ under 20 mm; medium ⫽ 20–59 mm; large ⫽ 60 mm and over. Class GASTROPODA Cuvier, 1797 Order CAENOGASTROPODA Cox, 1959 Superfamily AMPULLAROIDEA Gray, 1842 Family VIVIPARIDAE Gray, 1847 Subfamily VIVIPARINAE Gray, 1847 Genus VIVIPARUS Montfort, 1810 Type species.⎯Viviparus fluviorum Montfort, 1810 ⫽ viviparus (Linnaeus). Recent. VIVIPARUS MCBRIDEI new species Figure 4.1–4.3 Diagnosis.⎯Large-sized Viviparus, last whorl inflated, surface smooth. Description.⎯Shell large-sized, dextral; acutely pointed, protoconch unknown; teleoconch of four convex whorls; last whorl greatly inflated; sutures incised; aperture about one third of the total shell height; surface smooth. Etymology.⎯Dedicated to Dr. Earle F. McBride, The University of Texas at Austin, for his pioneer work and valuable contribution to the stratigraphy and understanding of the Difunta Group. Type.⎯Holotype IGM 3149, paratypes IGM 3150, IGM 3151. Holotype.⎯IGM 3149, height 48.0 mm, diameter 44.3 mm; paratypes IGM 3150, height 48.0 mm, diameter 41.3 mm; IGM 3151, height 43.5 mm, diameter 38.2 mm. Other material examined.⎯34 specimens. Occurrence.⎯Locality 3494, Puerto Colorado, Coahuila. Paleocene. Las Encinas Formation.

Discussion.⎯The Mexican specimens are not like any other species described from the Paleocene of North America. The most similar species is Viviparus raynoldsanus (Meek and Hayden, 1861, p. 446), from the Paleocene of Lower fork of Powder River, Montana. The Mexican specimens are larger, and not having an

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elevated spire and does not present the distinct revolving lines. It also differs in not having its whorls usually a little compressed around the middle, so as to give them a slightly shouldered appearance above. Also Bellamya ecclesi (Crowley and Pain, 1964, p. 51, fig. 21b), from Lake Malawi, Africa, differs in that last whorl is very inflated and presents also an acutely pointed spire. Bellamya, which lives in Africa and southern Asia, and Viviparus, which is more widely distributed in Europe, northern Asia, and North America, have very similar shells, and separation of the two genera is based on anatomical characteristics that are lacking in fossils. In addition to disparate geographical distributions, their presently recognized time distributions differ. Viviparus is reported from Upper Cretaceous in Europe, Asia, North and South America whereas Bellamya has not been recognized in the Cretaceous or Early Tertiary but has been identified in the Pleistocene in Africa and southern Asia. VIVIPARUS cf. V.

(Meek and Hayden, 1856) Figure 4.4, 4.5

LEIDYI

Description.⎯Shell medium-sized; protoconch unknown; teleoconch of five whorls, convex, last one elongate; sutures linear; aperture ovate; surface smooth. Material examined.⎯Four specimens. IGM 3152, height 47.3 mm, diameter 26.4 mm; IGM 3153, height 38.0 mm, diameter 24.7 mm. Occurrence.⎯Locality 3494, Puerto Colorado, Coahuila. Paleocene. Las Encinas Formation.

Discussion.⎯The Mexican specimens are similar in size to Viviparus leidyi (Meek and Hayden, 1856, p. 123), which is known with certainty only in its type locality 16 km below Fort Union, North Dakota, from the Paleocene Fort Union Formation (Hartman, 1984). They differ in that they are less convex and the last whorl is elongate, not convex. VIVIPARUS cf. V.

(Meek and Hayden, 1861) Figure 4.6, 4.7

RAYNOLDSANUS

Description.⎯Shell medium-sized; protoconch unknown; teleoconch of five whorls, convex, last one abruptly rounded; suture deep; aperture subovate; surface smooth. Material examined.⎯Five specimens. IGM 3154, height 38.4 mm, diameter 27.6 mm; IGM 3155, height 35.4 mm, diameter 32.3 mm. Occurrence.⎯Locality 3494, Puerto Colorado, Coahuila. Paleocene, Las Encinas Formation.

Discussion.⎯The first whorls of the teleoconch of the Mexican specimens are similar to those of Viviparus raynoldsanus (Meek and Hayden, 1861, p. 446) from the Paleocene of the Fort Union Formation, Powder River, Montana but differ in that the last whorl is more inflated; too, they are smaller. VIVIPARUS cf. V. AUSTRALIS Kues, 1995 Figure 4.8, 4.9 Description.⎯Shell small-sized, trochiform; protoconch unknown; teleoconch of four whorls, convex; whorls not shouldered; suture impressed; surface smooth; aperture not preserved. Material examined.⎯16 specimens. IGM 3156, height 29.9 mm, diameter 23.3 mm; IGM 3157, height 18.1 mm, diameter 16.0 mm. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian. Cerro del Pueblo Formation.

Discussion.⎯The specimens from Coahuila differ from Viviparus australis Kues, 1995 (p. 147), from Corral de Enmedio, Sonora, Corral de Enmedio Formation in being smaller in size and lacking shouldered whorls. All the specimens are incomplete, and have larger teleoconch whorls. This taxon is comparable to Viviparus beaumontiana (Matheron, 1842, p. 224, pl. 37, figs. 23, 24) from the Maastrichtian and Danian of France. Both have the same size and general form but differ in that the Mexican specimens lack the spiral striae of the French specimens.

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PERRILLIAT ET AL.—FRESHWATER GASTROPODS FROM MEXICO Family THIARIIDAE Morrison, 1954 Subfamily MELANATRIINAE Morrison, 1954 Genus PACHYCHILUS Lea and Lea, 1850 Type species.⎯Pachychilus graphium (Morelet, 1849). Recent. Guatemala. Subgenus PACHYCHILOIDES White, 1895 Type species.⎯Pachychilus (Pachychiloides) cleburni (White, 1876). Bear River Valley, Wyoming, Bear River Formation. PACHYCHILUS (PACHYCHILOIDES) LAWTONI new species Figure 4.10, 4.11 Diagnosis.⎯Medium-sized Pachychilus. Strong, slightly curved axial ribs; thin spiral ribs. Description.⎯Shell medium-sized, high spiral slender, elongate; protoconch unknown; teleoconch of ten gently convex whorls; suture distinct; sculpture of strong, slightly curved axial ribs, which extend from suture to suture; spiral sculpture of thin ribs that override the axial ribs; aperture not preserved. Etymology.⎯Named for Dr. Timothy Lawton from the Institute of Tectonic Studies, New Mexico State University, who has made important contributions to the regional geology of the Difunta Group. Type.⎯Holotype IGM 3158, paratype IGM 3159. Holotype.⎯IGM 3158, height 20.3 mm, diameter 7.1 mm; paratype IGM 3159, height 18.8 mm, diameter 7.3 mm. Other material examined.⎯100 specimens. Occurrence.⎯Locality 3496, La Carroza, Nuevo Leoń. La Carroza Formation.

Discussion.⎯The Mexican specimens differ from Pachychilus (Pachychiloides) cleburni (White, 1876) (p. 197) from Bear River Formation of the Bear River valley in southeastern Wyoming in that they are smaller and have spiral sculpture. Genus MELANATRIA Bowdich, 1822 Type species.⎯Buccinum flumineum Gmelin, 1791. Recent. Madagascar. MELANATRIA cf. M.

Vega and Perrilliat, 1992 Figure 4.12, 4.13 YPRESIANA

MELANATRIA

WYOMINGENSIS

261 (Meek, 1873)

Figure 4.14 Melania (Goniobasis?) wyomingensis MEEK, 1873, p. 516. Melania larunda WHITE, 1876, p. 131. Melania wyomingensis Meek. WHITE, 1883, p. 95, pl. 28, fig. 6a, b. Pachymelania wyomingensis (Meek). TAYLOR, 1975, p. 373. Description.⎯Shell medium-sized, moderately convex; teleoconch of seven whorls; suture linear; sculpture in last whorls with a prominent, strong spiral rib, outward projecting with tubercles; upper whorls with crenulated longitudinal ribs, crossed by five spiral lines. Material examined.⎯14 specimens. IGM 3162, height 27.6 mm, diameter 12.1 mm. Occurrence.⎯Locality 3495, Rincoń Colorado, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are smaller than the holotype. This species has been reported from the Laramie Group at Black Buttes Station, Union Pacific Railroad, Wyoming. Subfamily PALUDOMINAE Boss, 1982 Genus PYRGULIFERA Meek, 1877 Type species.⎯Pyrgulifera humerosa White, 1882. Bear River Formation, Bear River Valley, Wyoming. ?PYRGULIFERA sp. Figure 4.15 Description.⎯Shell medium-sized, subfusiform; protoconch unknown; teleoconch of four whorls, angular and shouldered; suture defined; sculpture of oblique ribs; aperture subovate. Material examined.⎯Two specimens. IGM 3163, height 22.6 mm, diameter 17.5 mm. Occurrence.⎯Locality 3492, General Cepeda 2, Coahuila. Campanian. Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are not well preserved and are assigned with question to Pyrgulifera; they are similar in the shape of shell and the shouldered whorls; ribs are faint or swollen. Subfamily THIARINAE Troschel, 1857 Genus MELANOIDES Olivier, 1804 Type species.⎯Melanoides fasciolata Olivier, 1804 ⫽ tuberculata Mu¨ller, 1774. Recent. Algeria.

Description.⎯Shell medium-sized, turreted; protoconch unknown; teleoconch of four whorls, convex; sculpture of curved axials of moderate strength that extend from suture below almost to suture above; spiral sculpture of threads of subequal strength; aperture not preserved. Material examined.⎯12 specimens. IGM 3160, height 20.5 mm, diameter 10.3 mm; IGM 3161, height 18.0 mm, diameter 9.5 mm. Occurrence.⎯Locality 3494, Puerto Colorado, Coahuila. Paleocene, Las Encinas Formation.

Subgenus MELANOIDES s. str. MELANOIDES (MELANOIDES) YOLANDAE new species Figure 4.16–4.19 Diagnosis.⎯Small Melanoides, first whorls with two beaded spirals, last ones with three beaded spirals; no axial sculpture.

Discussion.⎯The specimens from Coahuila are similar to those described by Vega and Perrilliat (1992, p. 607, figs. 4.12, 4.14– 4.16, 4.18) from the middle portion of the Adjuntas Formation (lower Eocene), from Cerro La Carroza, Nuevo Leoń, but the ones from Coahuila are incomplete and not well preserved.

Description.⎯Shell small, slender, turreted; protoconch unknown; teleoconch of eight whorls; whorls profile straight; suture impressed; the first three whorls with two beaded spirals, subsequent ones with three beaded spirals, the anterior one less prominent; last whorl with eight spirals, only the upper three remain beaded, the others becoming nearly smooth; axial sculpture not preserved; aperture ovate.

← FIGURE 4—1–3, Viviparus macbridei n. sp.; 1, apertural view, holotype IGM 3149, ⫻1.1; 2, apertural view, paratype IGM 3150, ⫻1.0; 3, apertural view, paratype IGM 3151, ⫻1.05. 4, 5, Viviparus cf. V. leidyi (Meek and Hayden, 1856). 4, apertural view, IGM 3152, ⫻1.04; 5, abapertural view, IGM 3153, ⫻1.05. 6, 7, Viviparus cf. V. raynoldsanus (Meek and Hayden, 1861). 6, abapertural view, IGM 3154, ⫻1.05; 7, abapertural view, IGM 3155, ⫻0.95. 8, 9, Viviparus cf. V. australis Kues, 1995. 8, apertural view, IGM 3156 ⫻1.05; 9, abapertural view, IGM 3157, ⫻1.5. 10, 11, Pachychilus (Pachychiloides) lawtoni n. sp.; 10, abapertural view, holotype IGM 3158, ⫻1.5; 11, apertural view, paratype IGM 3159, ⫻1.6; 12, 13, Melanatria cf. M. ypresiana Vega and Perrilliat, 1992. 12, abapertural view, IGM 3160, ⫻1.4; 13, apertural view, IGM 3161, ⫻1.5. 14, Melanatria wyomingensis (Meek, 1873). Abapertural view, IGM 3162, ⫻1.0. 15, ?Pyrgulifera sp., abapertural view, IGM 3163, ⫻1.04. 16–19, Melanoides (Melanoides) yolandae n. sp.; 16, abapertural view, holotype IGM 3164, ⫻2.6; 17, apertural view, paratype IGM 3165, ⫻2.6; 18, abapertural view, paratype IGM 3166, ⫻3.5; 19, abapertural view, paratype IGM 3167, ⫻3.2. 20– 22, Melanoides (Melanoides) wollebeni n. sp.; 20, abapertural view, holotype IGM 3168, ⫻2.0; 21, abapertural view, paratype IGM 3169, ⫻2.2; 22, apertural view, paratype IGM 3170, ⫻1.9. 23, 24, Tympanotonus (Tympanotonus) cretaceus (Wade, 1926). 23, abapertural view, IGM 3171, ⫻1.0; 24, abapertural view, IGM 3172, ⫻1.0. 25, 26, Tympanotonus (Tympanotonus) nodosa (Wolleben, 1977). 25, abapertural view, IGM 3173, ⫻0.9; 26, abapertural view, IGM 3174, ⫻1.0. 27, 28, Physa cepedaensis n. sp.; 27, abapertural view, holotype IGM 3175, ⫻3.2; 28, apertural view, paratype IGM 3176, ⫻5.0. 29, Physa cf. P. michaudi Matheron, 1842. Abapertural view, IGM 3177, ⫻1.7. 30, Physa cf. P. usitata White, 1895. Abapertural view, IGM 3178, ⫻1.7. Specimens 1– 7, 12, 13 are from locality IGM 3494; 8, 9, 27–30 are from locality 3491; 10, 11 are from locality IGM 3496; 14 is from locality IGM 3495; 15–22, 25, 26 are from locality IGM 3492; 23, 24 are from locality IGM 3476.

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Etymology.⎯In honor to the late Yolanda de Leoń Da´vila, an enthusiastic amateur collector from Saltillo, Coahuila, whose keen observation in the field made possible the finding of many specimens here described. Type.⎯Holotype IGM 3164, paratypes IGM 3165, IGM 3166, IGM 3167. Holotype.⎯IGM 3164, height 15.7 mm, diameter 4.6 mm; paratypes IGM 3165, height 11.8 mm, diameter 4.3 mm; IGM 3166, height 10.5 mm, diameter 4.1 mm; IGM 3167, height 9.5 mm, diameter 3.3 mm. Other material examined.⎯200 specimens. Occurrence.⎯Locality 3492, General Cepeda 2, Coahuila. Campanian. Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are unlike any others described from the Campanian of North America. The most similar species is Melanoides langi Pilsbry and Bequaert (1927, p. 259, pl. 22, figs. 1, 2) from the Sand banks of the Tshopo River, near Stanleyville, Belgian Congo, Africa, but the Mexican specimens are more slender, do not have convex whorls, have only three noded spirals, and do not have the spiral below the prominent suture, beaded, forming a narrow horizontal ledge. This genus is present in the Paleocene of Europe, Asia and Africa and in the Recent in northern and southern Africa and Asia. MELANOIDES (MELANOIDES) WOLLEBENI new species Figure 4.20–4.22 Diagnosis.⎯Shell small-sized; profile slightly convex; sculpture of three beaded spirals; last whorl with four beaded spirals. Description.⎯Shell small-sized, turreted; protoconch unknown; teleoconch of ten to 12 whorls; whorl profile slightly convex; suture impressed; sculpture of three beaded spiral cords in every whorl, between the first and second spiral cord a fine spiral thread is present; last whorl with four beaded spiral cords and two smooth spiral threads in the base; aperture ovate. Etymology.⎯Dedicated to James A. Wolleben, pioneer of the paleontological studies of the Difunta Group. Type.⎯Holotype IGM 3168, paratypes IGM 3169, IGM 3170. Holotype.⎯IGM 3168, height 12.1 mm, diameter 4.5 mm; paratypes IGM 3169, height 11.2 mm, diameter 4.7 mm; IGM 3170, height 11.3 mm, diameter 4.8 mm. Other material examined.⎯54 specimens. Occurrence.⎯Locality 3492, General Cepeda 2, Coahuila. Campanian. Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are unlike any others described from the Campanian in North America. The specimens differ from Melanoides anomala bukamana Pilsbry and Bequaert (1927, p. 258, pl. 21, fig. 13) from Lualaba River at Bukama, Africa, in that the African species has four spiral furrows (on the last three whorls), with the upper furrow a little more pronounced; and on the base three or four spiral cords whereas the Mexican ones have only three spiral cords on all spire whorls. The Mexican specimens are larger than those from Africa and have a larger shell. Superfamily CERITHIOIDEA Fe´russac, 1819 Family POTAMIDIDAE Adams and Adams, 1854 Genus TYMPANOTONUS Schumacher, 1817 Type species.⎯Tympanotonus fluviatilis Schumacher, 1817. Recent. Angola. Subgenus TYMPANOTONUS Schumacher, 1817 TYMPANOTONUS (TYMPANOTONUS) CRETACEUS (Wade, 1926) Figure 4.23, 4.24 Melanatria cretacea WADE, 1926, p. 158, pl. 55, fig. 1a, 1b, 2; SOHL, 1960, p. 79. Tympanotonus (Tympanotonus) cretaceus Wade. DOCKERY, 1993, p. 46, pl. 6, fig. 1; pl. 7, fig. 4. Description.⎯Shell large; teleoconch of five whorls; suture impressed and slightly undulating; sculpture of prominent, broadly spaced axial ribs, covered with nine spiral lirae; base with lirae separated with narrow interspaces. Material examined.⎯Three specimens. IGM 3171, height 42.5 mm, diameter 20.1 mm; IGM 3172, height 32.4 mm, diameter 18.0 mm. Occurrence.⎯Locality 3476, Rancho La Soledad, Coahuila. Maastrichtian. Olmos Formation.

Discussion.⎯This species was described from Coon Creek Tongue of the Ripley Formation, Coon Creek, McNairy County, Tennessee, and recognized from Tupelo Tongue of Coffee Sand, Lee County, Mississippi (Dockery, 1993). TYMPANOTONUS (TYMPANOTONUS) NODOSA (Wolleben, 1977) Figure 4.25, 4.26 Cerithium nodosa WOLLEBEN, 1977, p. 394, pl. 3, fig. 8. Description.⎯Shell of medium size; turreted; protoconch unknown; teleoconch of eight whorls or more; the first three or four whorls smooth, later whorls with five prominent elongated nodes parallel to spiral; aperture not preserved. Material examined.⎯917 specimens. IGM 3173, height 29.5 mm, diameter 13.5 mm; IGM 3174, height 26.8 mm, diameter 13.4 mm. Occurrence.⎯Locality 3492, General Cepeda 2, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯This species has been cited by Wolleben (1977) from the Parras Basin, Coahuila. He reported no spiral sculpture on his weathered specimens, and only one of our specimens has an indication of spiral threads on the first whorl. Order BASOMMATOPHORA Schmidt, 1855 Superfamily LYMNAEOIDEA Rafinesque, 1815 Family PHYSIDAE Fitzinger, 1833 Genus PHYSA Draparnaud, 1801 Type species.⎯Physa fontinalis (Linnaeus, 1758). Recent. Ronneby, Sweden. PHYSA

new species Figure 4.27, 4.28 Diagnosis.⎯Small-sized Physa; relatively slender channeled sutures; smooth. CEPEDAENSIS

Description.⎯Shell small, slender; protoconch unknown; teleoconch of five whorls; sutures channeled; glossy; last whorl convex; aperture ovate. Etymology.⎯Refers to General Cepeda County, Coahuila. Many important localities of the Campanian Cerro del Pueblo Formation are found in this region. Type.⎯Holotype IGM 3175, paratype IGM 3176. Holotype.⎯IGM 3175, height 9.1 mm, diameter 5.3 mm; paratype IGM 3176, height 5.5 mm, diameter 2.6 mm. Other material examined.⎯Seven specimens. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯These specimens differ from Physa usitata White (1895, p. 47, pl. 6, figs. 8, 9) in being smaller and more slender. PHYSA cf. P.

Matheron, 1842 Figure 4.29

MICHAUDI

Description.⎯Shell small-sized, nucleus unknown, spire with five whorls, stout; last whorl inflated; sutures oblicuous, depressed; without sculpture. Material examined.⎯24 specimens. IGM 3177, height 17.5 mm, diameter 9.7 mm. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are comparable to Physa michaudi Matheron (1842, p. 218, pl. 36, figs. 17, 18) from the Maastrichtian of Simiane, France, but they are smaller and do not present ornamentation. Also they are similar in size to Physa michaudi Matheron (Fabre-Taxy, 1959, p. 33, pl. 1, fig. 34) from the Maastrichtian of Provence, France. PHYSA cf. P. USITATA White, 1895 Figure 4.30 Description.⎯Shell small; nucleus and first whorls unknown; remaining whorls three; last whorl slender; sutures channeled; surface glossy. Material examined.⎯Three specimens. IGM 3178, height 15.2 mm, diameter 9.4 mm. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

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FIGURE 5—1–4, Mesolanistes magnus n. sp.; 1, abapertural view, holotype IGM 3179, ⫻0.9; 2, apertural view, holotype IGM 3179, ⫻0.9; 3, abapertural view, paratype IGM 3180, ⫻1.1; 4, apertural view, paratype IGM 3180, ⫻1.1. 5–7, Mesolanistes murrayi n. sp.; 5, abapertural view, holotype IGM 3182, ⫻1.0; 6, abapertural view, paratype IGM 3183, ⫻1.0; 7, abapertural view, paratype IGM 3184, ⫻1.1. 8, 9, Bulinus cf. B. atavus White, 1877, 8, abapertural view, IGM 3185, ⫻1.0, 9, apertural view, IGM 3185, ⫻1.0. 10, 11, Bulinus sp. 1. 10, abapertural view, IGM 3186, ⫻1.8; 11, abapertural view, IGM 3187, ⫻2.2. 12, 13, Bulinus sp. 2, 12, apertural view, IGM 3188, ⫻2.1; 13, abapertural view, IGM 3188, ⫻2.1. 14, 15, Bulinus sp. 3, 14, apertural view, IGM 6611, ⫻1.7, 15, apertural view, IGM 6611, ⫻1.7. 16, 17, Bulinus sp. 4, 16 apertural view, IGM 6613, ⫻1.8; 17, abapertural view, IGM 6613, ⫻1.8. 18, 19, Gyraulus zoltani n. sp., apertural view, holotype IGM 6615, ⫻6. 19, abapertural view, holotype IGM 6615, ⫻6. All specimens are from locality IGM 3491, except 1–4 from locality IGM 3492.

Discussion.⎯The Mexican specimens differ from Physa usitata White (1895, p. 47, pl. 6, figs. 8, 9) from Albian Bear River Formation, Southwestern Wyoming, in that they are not so slender and the first whorls are larger. Genus MESOLANISTES Yen, 1945 Type species.⎯Mesolanistes cretaceus Yen, 1945. Upper Cretaceous, Utah. MESOLANISTES MAGNUS new species Figure 5.1–5.4 Diagnosis.⎯Large-sized Mesolanistes, with depressed spire; last whorl very inflated; rounded; ornamentation of transverse ribs. Description.⎯Shell large-sized, stout; nucleus of one and a half whorls; spire with two and a half whorls, depressed, and in some specimens involute; last whorl inflated to very inflated, shoulder developed by abrupt rounding of

last whorl near aperture; suture channeled; ornamentation of numerous sinuous transverse ribs in spire and last whorl; well marked lines of growth on whole surface. Etymology.⎯Related to the large size of the shell. Type.⎯Holotype IGM 3179, paratypes IGM 3180, IGM 3181. Holotype.⎯IGM 3179, height 61.1 mm, diameter 44.5 mm, paratype IGM 3180, height 50.0 mm, diameter 51.6 mm; IGM 3181, height 47.0 mm, diameter 53.1 mm. Other material examined.⎯50 specimens. Occurrence.⎯Locality 3492, General Cepeda 2, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯The specimens from Coahuila are most similar to Physa reesidei Stanton (1917, p. 319, pl. 83, figs. 12, 13) from Fruitland Formation, 27 km south of the San Juan River and 4.5 km east of the Chaco River, New Mexico, but are larger and possess transverse sculpture. They differ from Physa felixi White (1880, p. 84, pl. 22, fig. 1a) from the Laramie Group, valley of Crow Creek, Weld County,

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Colorado, in being larger and in lacking the numerous small obliquely triangular papillae in oblique rows on the whole shouldered portion on the last whorl. Mesolanistes magnus differs from Mesolanistes cretaceus Yen (1945, p. 2, figs. 1–4) from the upper Kaiparowits Formation, Utah (Upper Cretaceous) in having a more depressed spire. The specimens are the largest fossil freshwater gastropods ever reported from Mexico. MESOLANISTES MURRAYI new species Figure 5.5–5.7 Diagnosis.⎯Small Mesolanistes with moderate elevated spire; with fine axial ribs. Description.⎯Shell small to medium-sized, ovate; commonly with moderately elevated spire of three whorls but depressed in some specimens; suture incised; last whorl enlarging rapidly; aperture ovate; sculpture of fine axial ribs. Etymology.⎯Dedicated to Grover E. Murray, pioneer of geologic research in the Difunta Group, northeastern Mexico. Type.⎯Holotype IGM 3182, paratypes IGM 3183, IGM 3184. Holotype.⎯IGM 3182, height 29.3 mm, diameter 24.4 mm; paratypes IGM 3183, height 28.1 mm, diameter 25.0 mm; IGM 3184, height 23.3 mm, diameter 18.5 mm. Other material examined.⎯75 specimens. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯These specimens differ from Mesolanistes cretaceus Yen (1945, p. 2, figs. 1–4) from the upper Kaiparowits Formation, Utah, in being smaller and in having a more depressed spire. In some specimens sculpture is absent due to preservation. Family PLANORBIDAE Rafinesque, 1815 Subfamily BULININAE Fischer and Crosse, 1880 Genus BULINUS O. F. Mu¨ller, 1781 Type species.⎯Bulinus senegalensis O. F. Mu¨ller, 1781. Recent. Senegal. BULINUS cf. B. ATAVUS White, 1877 Figure 5.8, 5.9 Description.⎯Shell medium-sized, elongate; protoconch and first whorls unknown; remaining three whorls of teleoconch; moderately convex; suture distinct; surface smooth; aperture not preserved. Material examined.⎯Four specimens. IGM 3185, height 16.7 mm, diameter 9.2 mm. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are not complete but in general form are similar to B. atavus White (1877, p. 601; 1880, p. 46) from the Campanian Judith River Formation, Dog Creek Valley, Montana. Mexican specimens differ from this species in being smaller. BULINUS sp. 1 Figure 5.10, 5.11 Description.⎯Shell small, narrow; protoconch unknown and first whorls not preserved; four whorls of teleoconch remaining nearly flat, suture deep; surface smooth; aperture subovate, acutely angular above. Material examined.⎯Five specimens. IGM 3186, height 12.0 mm, diameter 7.4 mm; IGM 3187, height 12.1 mm, diameter 7.1 mm. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are smaller than Bulinus longiusculus Meek and Hayden (1876, p. 541, pl. 43, fig. 16a, b) from the Paleocene Fort Union Group, 4.8 km below Fort Union, Dakota (Hartman, 1984, p. 1999), and the teleoconch whorls are larger. BULINUS sp. 2 Figure 5.12, 5.13 Description.⎯Shell small; ovate; protoconch unknown; teleoconch of five whorls; first four moderately convex, last whorl convex, globose; suture deep; surface smooth; aperture ovate.

Material examined.⎯20 specimens. IGM 3188, height 10.4 mm, diameter 6.1 mm; IGM 6610, height 11.0 mm, diameter 6.1 mm. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are not like any other species described from the Upper Cretaceous in North America. BULINUS sp. 3 Figure 5.14, 5.15 Description.⎯Shell small-sized; elongate; protoconch unknown; teleoconch of four nearly flat whorls; suture distinct; surface smooth; aperture not preserved. Material examined.⎯Seven specimens. IGM 6611, height 13.0 mm, diameter 6.1 mm; IGM 6612, height 13.5 mm, diameter 5.3 mm. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯These specimens differ from the preceding ones in being smaller and more elongate. BULINUS sp. 4 Figure 5.16, 5.17 Description.⎯Shell small, elongate; protoconch unknown; teleoconch of five whorls, flat; suture chanelled; surface smooth; aperture not preserved. Material examined.⎯Six specimens. IGM 6613, height 13.1 mm, diameter 4.6 mm; IGM 6614, height 11.0 mm, diameter 5.4 mm. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens differ from all described before in being smaller and having a less convex whorl profile. Subfamily PLANORBINAE Rafinesque, 1815 Genus GYRAULUS Charpentier, 1837 Type species.⎯Gyraulus hispidus Draparnaud, 1801. Recent. GYRAULUS ZOLTANI new species Figure 5.18, 5.19 Diagnosis.⎯Minute Gyraulus, with sculpture of very fine axial lines. Description.⎯Shell minute, discoidal, of four regularly rounded whorls; suture well defined; sculpture of very fine axial lines, abapertural side flattened; apertural side umbilicate; aperture oblique. Etymology.⎯To Dr. Zoltan de Cserna Gombos, prominent geologist and pioneer of tectonic studies or northeastern Mexico. Type.⎯Holotype IGM 6615, paratype IGM 6616. Holotype.⎯IGM 6615, height 0.3 mm, diameter 2.75 mm; paratype IGM 6616, height 0.5 mm, diameter 3.0 mm. Other material examined.⎯Four specimens. Occurrence.⎯Locality 3491, General Cepeda 1, Coahuila. Campanian, Cerro del Pueblo Formation.

Discussion.⎯The Mexican specimens are similar to Planorbis (Gyraulus) praecursoris White (1895, p. 46, pl. 6, figs. 4–7) from the Albian River Formation, near the mouth of Sulphur Creek and 3.2 km north of Cokeville, Wyoming, but they differ in being smaller in size. Also the Mexican specimens are similar to Planorbis (Gyraulus) militaris White (1880, p. 159) from the upper part Wasatch or Green River Formations (Eocene), from the head of Soldier’s Fork, Utah, but differ in that this last species although depressed is not flattened and also is larger. ACKNOWLEDGMENTS

We wish to express our gratitude to L. R. Saul (Natural History Museum of Los Angeles County), J. H. Hartman, and an anonymous reviewer for their valuable comments and generous efforts in providing elements to improve the manuscript. A. Altamira, Taller de Fotografia, helped with the photographs of specimens. Special thanks to PASAC (Paleonto´logos Aficionados de Sabinas, Asociacion Civil), for information on localities and specimens. Jose´ Manuel Padilla provided support with some figures. M. Pacheco helped with statistical analysis.

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